FORMATION OF ZEOLITE FROM THE SYSTEM NA2O-AL2O3-SIO2-H2O IN ALKALINE-MEDIUM (PH-GREATER-THAN-10)

The hydrothermal reactions of three aluminosilicates, i.e., kaolinite, metakaolinite, and sodium aluminosilicate gel, with aqueous sodium hydroxide have been reexamined. The molar composition of the quaternary system Na2O-Al2O3-SiO2-H2O was maintained constant in the three systems. Kaolinite gave a partially rehydrated nonbasic hydroxysodalite, whereas zeolite 4A is the product from metakaolinite and gel. The kinetics of the reaction and the crystal habit of the zeolite formed in the metakaolinite-alkali system appear to be different from that of conventional gel route of synthesis. The metakaolinite slowly dissolves in alkali to form a gel that is the direct precursor of zeolite 4A. However, the crystallization seems to start even before the metakaolinite gel conversion is completed. The ion-exchange and water-adsorption capacity and the chemical assay of the precursors and products have been used to substantiate the XRD and spectroscopic (i.r. and MAS n.m.r.) data in establishing the mechanism of the reaction.